Modern data communications technologies have greatly expanded the ability to communicate large amounts of data over many types of communications facilities. This explosion in communications capability not only permits the communications of large databases, but has also enabled the real-time (and beyond) digital communications of audio and video content. This high bandwidth communication is now carried out over a variety of facilities, including telephone lines (fiber optic as well as twisted-pair), coaxial cable such as supported by cable television service providers, dedicated network cabling within an office or home location, satellite links, and wireless telephony.
Each of these conventional communications facilities involves certain limitations in their deployment. In the case of communications over the telephone network, high-speed data transmission, such as that provided by digital subscriber line (DSL) services, must be carried out at a specific frequency range so as to not interfere with voice traffic, and is currently limited in the distance that such high-frequency communications can travel. Of course, communications over “wired” networks, including the telephone network, cable network, or a dedicated network, requires the running of the physical wires among the locations to be served. This physical installation and maintenance is costly, as well as limiting to the user of the communications network.
Wireless communication facilities overcome the limitation of physical wires and cabling, and provide great flexibility to the user. Conventional wireless technologies involve their own limitations, however. For example, in the case of wireless telephony, the frequencies at which communications may be carried out are regulated and controlled. Furthermore, current wireless telephone communication of large data blocks, such as video, is prohibitively expensive, considering the per-unit-time charges for wireless services. Additionally, since it is common to have multiple users within a certain frequency range, wireless telephone communications are subject to interference among the various users within the nearby area. Radio frequency data communication must also be carried out within specified frequencies, and is also vulnerable to interference from other transmissions and sources of noise. Satellite transmission is also currently expensive, particularly for bi-directional communications (i.e., beyond the passive reception of television programming).
A relatively new technology that has been proposed for data communications is the optical wireless network. According to this approach, data is transmitted by way of modulation of a light beam, in much the same manner as in the case of fiber optic telephone communications. A photoreceiver receives the modulated light, and demodulates the signal to retrieve the data. As opposed to fiber optic-based optical communications, however, this approach does not use a physical wire for transmission of the light signal. In the case of directed optical communications, a line-of-sight relationship between the transmitter and the receiver permits a modulated light beam, such as that produced by a laser, to travel without the use of an optical fiber as a waveguide. Optical wireless communications is inherently secure because in order to snoop on the transmission, the transmission would need to be broken. A broken transmission link is readily detected.
It is contemplated that the optical wireless network according to this approach will provide numerous important advantages. First, high frequency light can provide high bandwidth, for example ranging from on the order of 100 mega-bits-per-second (Mbps) to several giga-bits-per-second (Gbps), when using conventional technology. Additionally, this high bandwidth need not be shared among users, when carried out over line-of-sight optical communications between transmitters and receivers. Without other users on the link, of course, the bandwidth is not limited by interference from other users, as in the case of wireless telephony. Modulation can also be quite simple, as compared with multiple-user communications that require time or code multiplexing to permit multiple simultaneous communications. Bi-directional communication can also be readily carried out according to this technology. Finally, optical frequencies are currently not regulated, and as such no licensing is required for the deployment of such networks.
These attributes of optical wireless networks make this technology attractive both for local networks within a building, and also for external networks between buildings. Indeed, it is contemplated that optical wireless communications may be useful in data communication within a room, such as for communicating video signals from a computer to a display device, such as a video projector.
The communications between two optical wireless links involves the light beams from one optical wireless link impinging upon a photodetector on the other optical wireless link. To successfully align the light beams, the optical wireless links undergo an alignment procedure that results in the nominal alignment of the light beams. Following the alignment procedure, a more fine scale procedure is used to fine-tune the alignment of the light beams. To finely position the light beams, a feedback control system is created using positional data (and/or positional commands) transmitted over the light beams. The data and/or commands issued by one optical wireless link are used to make adjustments to the position of the light beam originating at the other optical wireless link. The feedback control system may remain in use during normal operation to ensure optimal performance.
It will be apparent to those of ordinary skill in the art of the present invention that the precise alignment of the light beam between the optical wireless links is a critical factor in the proper operation of the optical wireless network. To facilitate the precise alignment of the light beam using the feedback control system when the two optical wireless links can be arbitrarily installed, a common basis needs to be established. If a common basis is not established between the communicating optical wireless networks, one optical wireless link may respond in a manner that is inconsistent with the intended directions of the other wireless link. For example, if the optical wireless links were installed such that they were 180 degrees out of alignment with each other, a very real possibility if one is installed on a wall and the other is installed on an opposite facing wall, then one optical wireless link's command to move the light beam in an upward direction would actually result in the other optical wireless link moving the light beam in a downward direction because it's upward direction is truly the first optical wireless link's downward direction.
Therefore, a need exists in the art for a way to generate a common basis to permit the exchange of information for use in accurate station orientation determination.